The present invention concerns a method of estimating the speed of relative movement of a transmitter and a receiver, in communication with one another, of a telecommunications system, such as a wireless cellular network. The transmitter (or the receiver) which is moving is accommodated in a mobile station and the receiver (or the transmitter) is accommodated in a base station of a telecommunications system.
In such a system, the telecommunications are performed by radio link, thus allowing the mobile stations to communicate while moving. However, the effect of movement of a mobile station in the process of communication is generation of a disturbance referred to as a Doppler shift which affects the transmission of the information. The degradation brought about by the Doppler shift is a direct function of the speed of movement of the mobile station.
For both the base station and the mobile station, the fact of knowing the speed of movement of the mobile station can make it possible to optimize the transmission parameters and adjust the signal processing functions used in the process of communication according to the movement of the mobile.
The aim of the invention is to propose a method which makes it possible to estimate the speed of movement of a mobile station of a telecommunications system by processing the signals transmitted between the transmitter and the receiver.
To that end, an estimation method according to the invention is characterised in that it consists of analyzing the variations, in the channel between the transmitter and the receiver, of the amplitude or of the power of the signal transmitted by the transmitter, and of deducing, from the said variations, the Doppler shift undergone by the carrier wave and from that, the said speed of movement. The said analysis consists advantageously of an evaluation of the statistical and/or spectral properties of the variations of the power of the said channel.
According to another characteristic of the invention, the said estimation method consists of determining, for one instant and for a predetermined number of delays, the autocovariance of the channel power taken at the said time and a time delayed with respect to the time by the said delay, of determining the delay for which the said autocovariance reaches its first minimum and of deducing the Doppler shift from the said delay.
According to another characteristic of the invention, it consists, from a sequence of N samples of the channel power, of determining a sequence of a predetermined number of estimates representing the derivative of the autocovariance of the channel power, of determining the delay for which the said derivative of the autocovariance is zero and of deducing the Doppler shift from the said delay.
According to another characteristic of the invention, the said estimation method consists of determining the power spectral density of the channel power in order to obtain a frequency spectrum of the said channel power, of determining, among the frequencies of the said spectrum, the highest frequency for which the said spectrum has a level greater than a predetermined level and of deducing the Doppler shift from the said frequency.
The present invention also concerns an estimation method as just described, of the type used in the receiver of a telecommunications system equipped with a power control system by means of which the said receiver sends, to the transmitter, a control signal TC(n) so that the said transmitter commands its transmission power in order that the power received by the said receiver is substantially equal to a required power. The said method then consists of estimating the channel power C(n) as a function of the received power R(n), by means of the following relationship:
C(n)≈R(n)xe2x88x921/(1xe2x88x92zxe2x88x921).[TC(nxe2x88x92x)]
n is the time variable, z the z-transform variable, and x the total delay between the time at which the signal transmitted by the transmitter is received by the receiver and the time at which the control signal is received by the transmitter.
The present invention also concerns an estimation method as just described, of the type used in the transmitter of a telecommunications system equipped with a power control system by means of which the said receiver sends, to the transmitter, a control signal TC(n) so that the said transmitter commands its transmission power in order that the power received by the said receiver is substantially equal to a required power. It then consists of estimating the derivative of the autocovariance Cov[C(n),C(n+i)] of the channel power by making it equal to the derivative of the autocovariance Cov[T(n),T(n+i)] of the power T(n) transmitted by the said transmitter.                     Cov        xe2x80x2            ⁡              [                              C            ⁡                          (              n              )                                ,                      C            ⁡                          (                              n                +                i                            )                                      ]              ≈                            R          T          xe2x80x2                ⁡                  [          i          ]                    ⁢              (        n        )              ≈                  1        N            ⁢              xe2x80x83            ⁢                        ∑                      j            =            n                                n            +            N            -            1                          ⁢                  xe2x80x83                ⁢                                            T              ⁢                              (                j                )                                      ·            Δ                    ⁢                      xe2x80x83                    ⁢                      T            ⁢                          (                              j                +                i                            )                                                      for      ⁢              xe2x80x83            ⁢      0        ≤    i    ≤          i      max      
where xcex94T(j+i)=T(j+i+1)xe2x88x92T(j+i).
The present invention also concerns an estimation method of the type used in the receiver of a telecommunications system which is not equipped with a power control system. It then consists of estimating the variations in the channel power as being equal to the variations in the power received by the said receiver.
The present invention also concerns an estimation method as just described and which is used between a fixed station and a mobile station communicating with one another in both an inbound direction and an outbound direction. It then consists, in order to determine the said Doppler shift, of estimating, using one of the methods described previously, the Doppler shift of each of the inbound and outbound channels and of combining the said shifts, for example, by a weighted summation.
The present invention also concerns a receiver of a telecommunications system provided to be capable of entering into communication with a transmitter, the said receiver having a unit for measuring the power it receives from a transmitter by means of a channel, a unit for comparing the said received power with a required power and for transmitting a control signal to the said transmitter so that it commands its transmission power so that the power received by the said receiver is substantially equal to the said required power.
According to another characteristic of the invention, the said receiver also has a unit for estimating the power of the said channel from the information concerning the received power and from the control signal, and a unit which, on the basis of the estimate of the channel power delivered by the unit, delivers a Doppler shift estimation signal.
The present invention also concerns a transmitter of a telecommunications system provided to be capable of entering into communication with a receiver, the said transmitter receiving, from the said receiver with which it is in communication, a control signal, and commanding its transmission power in order that the power received by the said receiver is substantially equal to a required power.
According to another characteristic of the invention, it has a unit which delivers a Doppler shift estimation signal on the basis of a signal relating to the power transmitted by the said transmitter.